Discharge lamp

ABSTRACT

An additive-type mercury discharge lamp includes an arc tube comprising a radiation-transmitting sealed tubular body having a pair of electrodes arranged therein and filled with a dischargesustaining filling including mercury, a rare gas and at least one halogen in elemental form or in the form of a metal halide. The electrodes are coated with at least one rare earth metal oxide, and the filling includes a substance which reacts with the oxide of the rare earth metal to convert the same to a halide.

United States Patent [1 1 Sugiura et al.

[ Oct. 21, 1975 DISCHARGE LAMP [75] Inventors: Minoru Sugiura, Tokyo; Kyou-Ichi Maseki, Gyohda, both of Japan [73] Assignee: Iwasaki Denki Kabushiki Kaisha,

Tokyo, Japan [22] Filed: May 3, 1974 [21] App]. No.: 466,647

[30] Foreign Application Priority Data May 10, 1973 Japan 48-51l73 [52] US. Cl. 313/229; 313/178; 313/218; 313/346 R; 313/225 [51] Int. Cl. H01J 61/18 [58] Field of Search 313/229, 218, 346 R, 225, 313/227, 178

[56] References Cited UNITED STATES PATENTS 3,530,327 9/1970 Zollweg et a1. 313/229 X Primary Examiner-R. V. Rolinec Assistant ExaminerDarwin R. Hostetter Attorney, Agent, or Firm-Frank J. Jordan [57] ABSTRACT 4 Claims, 2 Drawing Figures US. Patent 0a. 21, 1975 3,914,636

Fig.1

I I I I I v I I000 2000 3000 4000 5000 e000 LIGHTING TIME (hr) DISCHARGE LAMP BACKGROUND OF THE INVENTION The present invention relates to a so-called additive type mercury discharge lamp, wherein, in addition to mercury and a rare gas, at least one kind of a suitable luminous metal and a halogen in elemental form or in the form of a metal halide are filled and sealed in the transparent arc tube positioned in the-discharge lamp.

The additive-type mercury discharge lamp is superior to the usual high pressure mercury vapor discharge lamp in its luminous efficiency and colour rendering and has become an important lighting source for general illumination purposes.

Such an additive-type mercury discharge lamp, wherein sodium, scandium, thorium and a halogen in elemental form or in the form of a metal halide are filled and sealed in an arc tube together with mercury and a rare gas is well known in the art. This lamp has the advantage that an excellent colour rendering can be obtained owing to the luminescence of the scandium filled in the arc tube, but requires a relatively high starting voltage.

For reducing the starting voltage, it has been proposed to apply thorium oxide to each electrode, as an electron emissive material. Such application of thorium oxide, however, lowers the lumen maintenance factor of the lamp. Further, such discharge lamp has serious disadvantages in that a partial transparency loss of the arc tube and a deformation or loss of the tip end of each electrode gradually occur due to the metallic scandium filled in the arc tube and the thorium oxide applied to the electrodes as the electron emissive material.

SUMMARY OF THE INVENTION It has now been found that, in such a discharge lamp containing sodium, scandium, thorium and a halogen, for instance, iodine, in elemental form or in the form of a metal halide, in addition to mercury and a rare gas, the thorium oxide applied to each electrode and the metallic scandium filled in the arc tube bear a close relationship to the disadvantages referred to. These disadvantages are overcome in accordance with the present invention by applying scandium oxide to the electrodes in lieu of thorium oxide'and by utilizing metallic thorium in the arc tube in lieu of scandium. Such a discharge lamp not only has a high lumen maintenance factor but also exhibits substantially no partial transparency loss of the arc tube or deformation or wear of the electrode tip ends. Further, a luminous spectrum analysis of the present discharge lamp shows the presence of scandium, despite the fact that scandium is not sealed therein in its elementary state. This result is due to the fact that the scandium oxide applied to the electrodes reacts with the thorium halide formed during operation of the lamp to form scandium halide, as shown in the following equation, which halide contributes to the total luminescence of the lamp:

2 Sc O 3 ThX 4 ScX 3 ThO wherein X represents a halogen atom, for instance, iodine.

Scandium oxide as the substance applied to the electrodes may be substituted by one or more rare earth metal oxides. A mixture of scandium oxide and at least one other rare earth metal oxide can also be used for this purpose.

Metallic thorium filled in the sealed arc tube may be:

substituted by aluminum or silicon, or a mixture of any two or all three thereof. 'If aluminum or silicon is solely used, a somewhat higher starting voltage is required. In this case, the required starting voltage can be reduced by filling a small amount of metallic thorium in addition thereto. g k

A principal object ofthe present invention, therefore, is to provide an additive type mercury discharge lamp which obviates and overcomes the disadvantages of the conventional one referred to above.

A specific object of the present invention is to provide an additive type mercury discharge lamp, which can be operated with a relatively low starting voltage and shows a relatively high lumen maintenance factor, a less partial transparency loss of the arc tube and a less deformation or wear of the electrode tip ends in comparison with any conventional discharge lamp of this type.

According to the present invention, the objects referred to and the other various objects to be appreciated by studying the specification can be attained by an additive type mercury discharge lamp, wherein the electrodes for the lamp are coated with at least one rare earth metal oxide and a substance that reacts with said oxide, during operation of the lamp, to form a halide of the rare earth metal is filled in the arc tube.

BRIEF DESCRIPTION OF THE DRAWING The present invention can be understood more clearly by referring to the accompanying drawing, in

which 1.

FIG. 1 is a longitudinal cross section of an-additi ve type mercury discharge lamp according to the present invention; and

FIG. 2 is a graph showing thelife performance curves of the discharge lamp according to the present invention and of a conventional one of this type. i

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, an additive type mercury discharge lamp according to the present invention is generally designated by reference numeral 10. The arc tube 10 includes a radiation transmitting sealed tubular body 12 of a transparent heat-resisting material, such as quartz glass, a pair of electrodes 14, 16 arranged oppositely at both ends of said tubular body lead-out wires 18, 20, connectors 22, 24 for connecting the electrodes l4, 16 to the lead-out wires 18. 20, respectively and being made of a metallic foil, for instance, molybdenum foil, and lagging members 26, 28 mounted on the end parts of the body so as to surround same near the electrodes l4, 16 from the outside of the tubular body 12. Both ends of the body 12 are pinch-sealed, so that an outer end of each electrode connects electrically to an inner end of each lead-out wire through the respective connector. The structure per se of the discharge lamp is well known in the art, but electric power and dimensions and volume of a typical one are shown as follows: Power: 400 W Dimensions of the Arc Tube: Inner diameter: about 20 mm Distance between electrodes: about 45 mm Inner volume: 16 cc EXAMPLE 1 In such an arc tube as shown in FIG. 1 and as having the dimensions referred to, scandium oxide was applied to the electrodes l4, l6 and, in the tubular body 12, 7mg of mercury iodide, 20mg of sodium iodide, 2mg of thorium and argon gas (filling pressure: 20 mmI-Ig) were filled and the lamp was then sealed. The resulting lamp was tested by lighting the same and obtained the following results. During the operation, there was found substantially no partial transparency loss of the lamp tube or deformation or wear of tip end of each electrode.

Initial luminous flux: 34,000 to 37,000 lm Starting voltage: 200 to 250 V Average colour rendering index (Ra): 62 to 68 Lumen maintenance factor (after 6,000 hours):

about 80% As a control, a conventional discharge lamp was manufactured, by applying thorium oxide to the electrodes and by using an arc tube similar to that illustrated in FIG. 1 and sealing in the arc tube 7mg of mercury iodide, 20mg of sodium iodide, 0.5mg of scandium, 1mg of thorium and argon gas (sealing pressure: 20 mmI-Ig), and then tested to obtain the following results:

Initial luminous flux: 34,000 to 37,000 lm Starting voltage: 200 to 250 V Average colour rendering index (Ra): 60 to 66 Lumen maintenance factor (after 6,000 hours):

about 73% During the operation of this control lamp, substantial partial transparency loss and deformation or wear of the tip end of each electrode occurred.

FIG.. 2 shows curves of the life performances of the discharge lamp according to the present invention and the conventional discharge lamp referred to hereinabove as control". Life performance curve A of the discharge lamp according to the present invention shows no sudden reduction in total luminous flux during operation, but curve B of the conventional discharge lamp shows a sudden reduction in total luminous flux at the initial stage of its operation.

EXAMPLE 2 In a tubular body as shown in FIG. 1 and having dimensions as referred to, mg of mercury iodide, 2mg of thallium iodide, 2mg of throium and argon gas (billing pressure: mmHg) were sealed, after having applied dysprosium oxide to each electrode. An additive type mercury discharge lamp (400 W) which comprises the resulting arc tube was tested by lighting the same to obtain the following results:

Initial luminous flux: 31,000 to 33,000 1m Starting voltage: 210 to 250 V Average colour rendering index (Ra): .78 to Lumen maintenance factor (after 6,000 hours):

about 78% During the operation, there was found neither partial transparency loss of the tubular body nor deformation of the electrodes.

EXAMPLE 3 Various additive type mercury discharge lamps were manufactured as in theprecedingExamples, excepting that, as the substance applied to each electrode, a rare earth metal oxide other than scandium and dysprosium oxides, such as an an oxide of yttrium, holmium, thulium, or erbium, or a mixture of rare earth metal oxides was used for each lamp and excepting that, as the substance reacting with such oxideor oxides, thoriumaluminum, thorium-silicon, throium-aluminum-silicon, aluminum, silicon and aluminum-silicon were used for each lamp. The lamps were tested to obtain results similar to those obtained in Examples 1 and 2.

We claim:

1. An additive-type mercury discharge lamp which includes an arc tube comprising a radiationtransmitting sealed tubular body, a pair of electrodes respectively arranged at the opposite ends of said body, a lead-out conductor connected electrically to each electrode, and a discharge-sustaining filling of preselected materials in predetermined amounts enclosed in said body and including mercury, a rare gas, and at least one halogen in elemental form or in the form of a metal halide, at least one of said electrodes being coated with at least one rare earth metal oxide, and said filling also including a substance that reacts with said rare earth metal oxide, during operation of the lamp, to form a halide of said rare earth metal.

, 2. A discharge lamp as claimed in claim 1, wherein said rare earth metal oxide is selected from the group consisting of the oxides of scandium, yttrium, and dysprosium.

3. A discharge lamp as claimed in claim 1, wherein said substance is selected from the group consisting of thorium, aluminum, silicon, thorium-aluminum, thorium-silicon, aluminum-silicon, and thorium-aluminumsilicon.

4. A discharge lamp'asclaimed in claim 1, wherein said rare earth metal oxide comprises scandium oxide and said substance comprises thorium. 

1. AN ADDITIVE-TYPE MERCURY DISCHARGE LAMP WHICH INCLUDES AN ARC TUBE COMPRISING A RADIATION-TRANSMITTING SEALED TUBULAR BODY, A PAIR OF ELECTRODES RESPECTIVELY ARRANGED AT THE OPPOSITE ENDS OF SAID BODY, A LEAD-OUT CONDUCTOR CONNECTED ELECTRICALLY TO EACH ELECTRODE, AND A DISCHARGE-SUSTAINING FILING OF PRESELECTED MATERIALS IN PREDETERMINED AMOUNTS ENCLOSED IN SAID BODY AND INCLUDING MERCURY, A RARE GAS, AND AT LEAST ONE HALOGEN IN ELEMENTAL FORM OR IN THE FORM OF A METAL HALIDE, AT LEAST ONE SAID ELECTRODES BEING COATED WITH AT LEAST ONE RARE EARTH METAL OXIDE, AND SAID FILLING ALSO INCLUDING A SUBSTANCE THAT REACTS WITH SAID RARE EARTH METAL OXIDE, DURING OPERATION OF THE LAMP, TO FORM A HALIDE OF SAID RARE EARTH METAL.
 2. A discharge lamp as claimed in claim 1, wherein said rare earth metal oxide is selected from the group consisting of the oxides of scandium, yttrium, and dysprosium.
 3. A discharge lamp as claimed in claim 1, wherein said substance is selected from the group consisting of thorium, aluminum, silicon, thorium-aluminuM, thorium-silicon, aluminum-silicon, and thorium-aluminum-silicon.
 4. A discharge lamp as claimed in claim 1, wherein said rare earth metal oxide comprises scandium oxide and said substance comprises thorium. 